This invention relates to a transducer element configuration for an ultrasound transducer array.
FIG. 1 depicts a conventional filled 2D ultrasound transducer array 10. The array 10 includes a plurality of rows 12 of transducer elements 14. Each element 14 is formed of a piezoelectric material. Each row 12 extends along an azimuthal ("x") direction. The rows 12 are parallel and spaced orthogonally in an elevation ("y") direction. The elements 14 receive high frequency electronic signals and convert the electronic power into mechanical ultrasound energy. The elements 14 emit ultrasound signals in a generally orthogonal depth ("Z") direction. Conventionally, the elements 14 serve as both transmitting elements and receiving elements. During operation, the function of elements 14 periodically switches between transmission and receptzon. Thus, the transmitting elements and receiving elements are "coincident, " "time shared" elements.
For a conventional 1D array the transducer array 10 includes only one row 12 of elements 14. Transmit and receive elements are coincident, time-shared elements 14. The final beam-pattern is defined along one dimension (e.g., the length of the row). To meaningfully scan a patient's anatomy with an ultrasound transducer array, the arrays final beam-pattern is controlled using beam-forming parameters. In particular, the number of transducer elements and the transmit and receive profiles among the elements are controlled. The net wave pattern of transmitted and reflected ultrasound signals defines the final beam-pattern. The beam-forming control parameters include: aperture, apodization, focus and steering. Aperture is a control of the number of active elements 14 along the azimuth. Apodization is a voltage weighting profile of the active elements 14. Focus is a time delay profile of such weighting. Steering is a control of focus "depth" point(s) along the azimuth. In a 1D array azimuthal aperture, apodization, focus and steering parameters serve as the beam-forming control parameters.
The quality of focus in an ultrasound system is physically limited by the size of a transducer array's aperture. Typically, this aperture is subdivided into many transducer elements along the azimuth direction of the array. Amplitude and phase of transmitted and received signals at each element are individually adjusted. Each active element in such a design requires a separate set of electronics to adjust the gain and delay of individual signals prior to beam-forming and focussing. As many as 128 such channels are typical to generate focus along the azimuth.
Overall image quality is affected not only by azimuthal focus, but also by elevational focus. A fixed focus lens is generally attached to the face of a 1D transducer array to provide for a single focus along the elevation. This does not allow for electronic control of the elevational focus as a function of distance from the transducer face. One solution is to subdivide the transducer array into elements, not only along the azimuth, but also along the elevation to form a two-dimensional array with each element having its own electronics.
Such a 2D structure having electronics for each transducer elements adds processing overhead and cost to the ultrasound system. Both of these are concerns in the ultrasound field. Ultrasound imaging typically is a processing intensive operation. Specifically, each channel (i.e., independent element of a transducer array) provides ultrasound vector data to be processed at a given sampling rate. Data from the multiple channels is used to create an image. Typically, a large number of channels enhances resolution and image quality. However, as the number of channels increases, the throughput requirements for imaging also increase. Thus, the types of algorithms or other vector/image processes performed on input data typically is limited by a system's throughput capability. Accordingly, it is desirable to reduce the number of channels or otherwise ease the processing burden while maintaining or improving image quality.
With regard to cost, it is desirable to reduce cost while maintaining or improving performance or while redefining performance needs.
To reduce the cost and complexity of the two-dimensional array Defranould and Souquet (1977) demonstrated a bidimensional array in which one focuses in one direction (e.g., azimuth) on transmit and in the other direction (e.g., elevation) on receive. This was accomplished by centrally locating a column of transducer elements subdivided in elevation between rows of elements subdivided laterally. A shortcoming of such array, however, is that mechanical scanning is needed to obtain data for a 2D image. Another approach is presented by Turnbull and Foster (1991) in which a sparse 2D array is formed. For a sparse 2D array, elements throughout the aperture are chosen at random to reduce the number of channels of electronics. As a result, focus along the elevation and azimuth occurs at the expense of other performance criteria. Specifically, side lobe levels increase and transducer gain decrease in an undesirable manner.
In U.S. Pat. No. 5,278,757, Hoctor and Kassam describe a synthetic aperture system using a reduced redundancy phased array. They describe an array with amplitude weightings and non-uniformly spaced ultrasonic transducers during transmit and receive modes. A number of component images are formed which when added together define a single image formed by a scanned beam of uniformly spaced transducers with the designated aperture. Azimuthal focussing is described. A shortcoming of this approach is that multiple images are needed to form a final visually displayed image. Accordingly there is a need for a 2D array structure which improves elevational focus for real-time imaging applications without significant throughput and cost penalties.